A silicoaluminophosphate, also referred to herein as SAPO, are largely composed of silicon, aluminum, phosphorus and oxygen and may have a 3-dimensional microporous crystal framework structure of PO2+, AlO2− and SiO2 tetrahedral units. The cages, channels and cavities created by the crystal framework of these materials permit their use in applications for the separation of mixtures of molecules based on their effective sizes.
SAPO membranes are known to be useful in gas separations. U.S. Published Application 2007/0265484 describes a method for making SAPO-34 membranes on porous supports. These membranes may be used for the separation of carbon dioxide and methane. The method involves applying a limited quantity of SAPO-34 crystalline material to at least part of the surface of a porous support in the form of loose crystals and then rubbing them onto and/or into the surface for the purpose of “seeding” the surface with SAPO-34 crystals in preparation for membrane growth. A continuous SAPO-34 membrane layer is then grown on the surface using the secondary growth technique. In secondary growth, an aqueous SAPO-34 forming gel comprising of silicon, aluminum, phosphorus, oxygen and organic templating agent(s) is prepared and the porous support is contacted with this gel. Next, the porous support and the gel are heated to form a thin layer of SAPO-34 crystalline material on the support. Finally, the supported SAPO layer is calcined to remove the templating agent(s). U.S. Published Application 2007/0265484 is herein incorporated by reference in its entirety.
The crystals that are added to the surface of the support are believed to act as crystallization nuclei for the synthesis mixture of SAPO-34 gel during hydrothermal treatment. Crystals that act as crystallization nuclei may be referred to as “seed crystals”. The crystals applied to the support may be referred to generally as seed crystals even though every single crystal applied to the support need not act as a nucleus for subsequent crystallization. During membrane synthesis, growth of the seed crystals, nucleation of new crystals on the seed crystals, and even complete dissolving of the seed crystals may occur.
The process for seeding by rubbing the seed crystals onto and/or into the support does produce functional membranes but this process is difficult to apply on a large and commercial scale. Thus, it would be advantageous if a new seeding method could be found that can be scaled up to a commercial scale operation while still providing the same or better functional membranes with good selectivity for carbon dioxide over methane and good carbon dioxide permeance.